The publications and other materials used herein to illuminate the background of the invention and in particular cases to provide additional details respecting its practice are incorporated herein by reference and for convenience are numerically referenced in the following text and respectively grouped in the appended bibliography.
Human interferons can be classified in three groups on the basis of different antigenicity and biological and biochemical properties. The first group comprises a family of leukocyte interferons which are normally produced mainly by constituent cells of human blood upon viral induction. These have been microbially produced and found to be biologically active (1,2,3). Their biological properties have prompted their use in the clinic as therapeutic agents for the treatment of viral infections and malignant conditions (4).
In the second group is human fibroblast interferon, normally produced by fibroblasts upon vital induction, which has likewise been microbially produced and found to exhibit a wide range of biological activities (5). Clinical trials also indicate its potential therapeutical value. The leukocyte and fibroblast interferons exhibit very clear similarities in their biological properties despite the fact that the degree of homology at the amino acid level is relatively low. Both groups of interferons contain from about 165 to about 166 amino acids and are acid stable proteins.
Human gamma interferon (also variously referred to as immune interferon, .gamma.-interferon, IIF or IFN-.gamma.) exhibits the antiviral and anti-proliferative properties characteristic of the interferons but, in contrast to leukocyte and fibroblast interferons, is pH 2 labile. Prior to the production of gamma interferons via recombinant DNA technology, it had been produced mainly upon mitogenic induction of lymphocytes. Human gamma interferon is clearly antigenically distinct from the leukocyte and fibroblast interferons. Gray, Goeddel and co-workers were the first to report expression of a recombinant gamma interferon (6), which has proven to exhibit the characteristic properties of human gamma interferon, i.e., anti-viral and anti-proliferative activity coupled with pH 2 lability. The recombinant gamma interferon of Gray and Goeddel, as produced in E. coli, consisted of 146 amino acids, the N-terminal portion of the molecule commencing with the sequence CYS-TYR-CYS-. Derynck and others subsequently reported (7) a further recombinant gamma interferon having the same N-terminus and a single amino acid substitution, the polypeptide perhaps constituting an allelic variant of that earlier reported in reference (6). Other workers have reported the production of still further recombinant gamma interferons in which one or more of the amino acids present in Goeddel and Gray's original publication (6) have allegedly been substituted.
For example, Alton et al. (17) report on a series of IFN-gammas wherein a single amino acid substitution at position 81 of the Gray et al. (6) gamma interferon resulted in an IFN-gamma that retained only 70 percent of the activity (on a relative basis) and wherein an additional deletion of the cys-tyr-cys at positions 1, 2, 3 of this IFN-gamma further reduced relative activity resulting in an IFN-gamma having only 49 percent of the Gray et al. (6) gamma interferon.
In our hands, recombinant gamma interferons whose N-terminal amino acid sequence comprises cysteine residues have proven problematic from the standpoint of oligomerization which may involve participation of sulfhydryl groups of one or more of the cysteine residues in disulfide bond formation. Our inability to completely reduce these putative disulfide linkages suggests the problem may be more complex, possibly also involving reaction through the hydroxyl function of the cysteine-bounded tyrosine residue. These recombinant interferons have proven somewhat unstable and, whether resulting from such instability or otherwise, have proven of less than optimal utility.